This invention pertains to flexible polyurethane foams which can be thermally bonded to substrates and more particularly to polyurethane foams formulated from polyisocyanates and polyether polyols.
It is known in the art that the urethane linkages of polyurethane foams are formed by the exothermic reaction of a polyfunctional isocyanate and a polyfunctional active hydrogen-containing compound in the presence of a catalyst and a blowing agent. In accordance with the "one-shot process" which is the most widely used industrial technique, direct reaction is effected between the polyisocyanate, and the active hydrogen-containing compound or compounds and is regulated by the catalyst system and surfactant.
It is also well known that suitable active hydrogen-containing compounds include polyether polyols and polyester polyols. However, it is equally well known that while flexible polyurethane foams formulated from polyester polyols have excellent flame lamination properties, those flexible polyurethane foams formulated from polyether polyols are unacceptable for flame lamination. Unfortunately the polyesters used in this application are much more expensive than the polyether polyols. This means that fabricators wishing to bond polyurethane foams derived from polyether polyols must resort to the use of an additional component, namely, an adhesive in order to bond flexible polyurethane foams derived from polyether polyols. The inconvenience and added expense of this procedure is obvious.
Two previous solutions to this problems have been provided but each introduces problems of its own which has left much room for improvement in this art. First of all J. C. Flanders in U.S. Pat. No. 3,205,120 issued on Sept. 7, 1965 describes the preparation of polyether polyol-based polyurethane foams suitable for fabrication of laminates by a heat sealing technique wherein the conventional commercial polyether polyol flexible polyurethane foam reaction mixture contained a minor amount of a polyol having a molecular weight of 200 to 1500. The preferred low molecular weight polyol was tris(dipropylene glycol) phosphite. Other low molecular weight polyols disclosed by Flanders include alkylene oxide adducts of polyhydroxy alkanes, phosphorus-containing acids, and the like. It was later found that there are processing difficulties encountered in the production of the Flanders polyether polyurethane foams that stem from the high reactivity of the polyether polyol mixture. These difficulties at least cancel the cost advantage of the polyether polyol based foam over polyester polyol based foam with the result that there has been no great incentive for laminate manufacturers to switch from polyester to polyether.
The second improvement described by F. E. Critchfield and R. D. Whitman in U.S. Pat. No. 3,497,416 issued on Feb. 24, 1970 comprises the use, in the production of heat-sealed polyurethane foam laminates, of polyurethane foam which is produced by reacting, in the presence of a foaming agent, a polyether polyol with the reaction product of an aromatic polyisocyanate and dipropylene glycol or dibutylene glycol. This reaction product, a quasi-prepolymer, is prepared by using an excess of an aromatic polyisocyanate over the amount of dipropylene glycol and/or dibutylene glycol used. While this disclosure constitutes a further advance in the art, it is still not completely satisfactory for commercial use by laminate manufacturers because in order to obtain an open foam one must incorporate a powerful cell opening agent, such as, dimethylformamide. The use of a cell opener is normally not used in polyurethane foam processing. In addition these polyurethane foams have very high compression sets which are unacceptable for most end use applications.